1. Field of the Invention
The present invention relates to an optical signal readout apparatus for a video long playing record in which the recorded signal is detected by a laser beam. 2. Description of the Prior Art
An example of a conventional optical signal readout apparatus is shown in block diagram form in FIG. 1. A rotary disc B having information recorded thereon is driven by an electric motor C, and the information is read out by a reading apparatus A. The information is recorded in the form of pits or depressions b arranged along a plurality of coaxial or spiralling tracks B1, B2, etc. on the disc B, as shown in FIG. 2. Each information pit b has some optically detectable property different from that of the area surrounding it, such as light reflectivity, light absorptivity or light refractivity, and/or has a different physical structure, such as a recess or land. The recorded information is represented by the specific repetition rate of the pits along the direction of the tracks B1, B2 etc., i.e., the wavelength or distance between adjacent pits, and/or the length of each pit.
A bundle of light beams emanating from a source 1, such as a helium or neon laser, is passed through a collimator 2 and a beam splitter 3 to a movable mirror 4 and a condenser lens 5. The condenser lens serves to focus the collimated light beam onto the rotary disc B as a fine spot.
When the disc B is driven by the motor C, the focused laser spot sequentially scans the pits b along, for example, track B2. Assuming that the pits have a reflectivity different from that of the surrounding areas, the light reflected back from the disc is gathered or collected by the condenser lens 5. The collected light is reflected by the movable mirror 4 and separated by the beam splitter 3. The separated light is directed onto a photo detector 6, from which an electrical signal representative of the information recorded by the scanned pits b is obtained.
The photo detector 6 comprises a pair of photo detecting elements 6a and 6b. The electrical signals corresponding to the reflected light beams incident on the respective detecting elements are amplified by amplifiers 7 and 8. Each of the amplified signals is supplied to both an adder 9 and a subtractor 10. The output of the adder 9, which is the sum of the outputs of the amplifiers 7 and 8, represents the information recorded on the rotary disc B, while the output of the subtractor 10 is employed as a tracking signal. That is, at a given instant during the scan of the information track B2, the reflected light pattern projected onto the photo detector 6 is as shown in FIG. 3. Since the brightness of the pits b differ from that of the surrounding area, the output of the adder 9 thus represents the recorded information signal.
When the center line of the information track B2 coincides with the boundary line separating the photo detecting elements 6a and 6b, as shown in FIG. 3, the output of the subtractor 10 will be zero. When, however, the boundary line is shifted with respect to the center line of the track, a difference signal will appear at the output of the subtractor 10 whose amplitude is proportional to the magnitude of the shift and whose polarity indicates the direction thereof. This difference signal is used to control the scanner tracking.
More specifically, the difference signal is supplied to a tracking control device 11. The low frequency component of the output signal from the tracking control device 11 is applied to an amplifier 12 as a coarse tracking signal, whose output drives a feeding device 13 to radially shift the reading apparatus A in a direction normal to the tracks on the rotary disc B. The high frequency component of the output signal from the tracking control device 11 is applied to an amplifier 14, whose output is supplied to a driving device 15 which controls the movable mirror 4 to implement fine tracking.
In the conventional readout apparatus described above, the detected information signal may include some irregularities or variations due to variations in the light intensity from the source 1 and/or variations in the reflectivity of the recording areas on the rotary disc B. The level of such variations may decrease with time due to the aging and degradation of the light source 1, but this in turn degrades the quality of the reproduced information. Further, the light intensity incident on the photo detector 6 may sometimes exceed an upper tolerance limit, and in such cases there is a possibility of burning the photo detecting elements 6a and 6b.